Re: CFQ idling kills I/O performance on ext4 with blkio cgroup controller

[Srivatsa S. Bhat]

On 5/23/19 10:22 AM, Paolo Valente wrote:
> 
>> Il giorno 23 mag 2019, alle ore 11:19, Paolo Valente <paolo.valente@xxxxxxxxxx> ha scritto:
>>
>>> Il giorno 23 mag 2019, alle ore 04:30, Srivatsa S. Bhat <srivatsa@xxxxxxxxxxxxx> ha scritto:
>>>
[...]
>>> Also, I'm very happy to run additional tests or experiments to help
>>> track down this issue. So, please don't hesitate to let me know if
>>> you'd like me to try anything else or get you additional traces etc. :)
>>>
>>
>> Here's to you!  :) I've attached a new small improvement that may
>> reduce fluctuations (path to apply on top of the others, of course).
>> Unfortunately, I don't expect this change to boost the throughput
>> though.
>>
>> In contrast, I've thought of a solution that might be rather
>> effective: making BFQ aware (heuristically) of trivial
>> synchronizations between processes in different groups.  This will
>> require a little more work and time.
>>
> 
> Hi Srivatsa,
> I'm back :)
> 
> First, there was a mistake in the last patch I sent you, namely in
> 0001-block-bfq-re-sample-req-service-times-when-possible.patch.
> Please don't apply that patch at all.
> 
> I've attached a new series of patches instead.  The first patch in this
> series is a fixed version of the faulty patch above (if I'm creating too
> much confusion, I'll send you again all patches to apply on top of
> mainline).
> 

No problem, I got it :)

> This series also implements the more effective idea I told you a few
> hours ago.  In my system, the loss is now around only 10%, even with
> low_latency on.
> 

When trying to run multiple dd tasks simultaneously, I get the kernel
panic shown below (mainline is fine, without these patches).

[  568.232231] BUG: kernel NULL pointer dereference, address: 0000000000000024
[  568.232257] #PF: supervisor read access in kernel mode
[  568.232273] #PF: error_code(0x0000) - not-present page
[  568.232289] PGD 0 P4D 0
[  568.232299] Oops: 0000 [#1] SMP PTI
[  568.232312] CPU: 0 PID: 1029 Comm: dd Tainted: G            E     5.1.0-io-dbg-4+ #6
[  568.232334] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/05/2016
[  568.232388] RIP: 0010:bfq_serv_to_charge+0x21/0x50
[  568.232404] Code: ff e8 c3 5e bc ff 0f 1f 00 0f 1f 44 00 00 48 8b 86 20 01 00 00 55 48 89 e5 53 48 89 fb a8 40 75 09 83 be a0 01 00 00 01 76 09 <8b> 43 24 c1 e8 09 5b 5d c3 48 8b 7e 08 e8 5d fd ff ff 84 c0 75 ea
[  568.232473] RSP: 0018:ffffa73a42dab750 EFLAGS: 00010002
[  568.232489] RAX: 0000000000001052 RBX: 0000000000000000 RCX: ffffa73a42dab7a0
[  568.232510] RDX: ffffa73a42dab657 RSI: ffff8b7b6ba2ab70 RDI: 0000000000000000
[  568.232530] RBP: ffffa73a42dab758 R08: 0000000000000000 R09: 0000000000000001
[  568.232551] R10: 0000000000000000 R11: ffffa73a42dab7a0 R12: ffff8b7b6aed3800
[  568.232571] R13: 0000000000000000 R14: 0000000000000000 R15: ffff8b7b6aed3800
[  568.232592] FS:  00007fb5b0724540(0000) GS:ffff8b7b6f800000(0000) knlGS:0000000000000000
[  568.232615] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  568.232632] CR2: 0000000000000024 CR3: 00000004266be002 CR4: 00000000001606f0
[  568.232690] Call Trace:
[  568.232703]  bfq_select_queue+0x781/0x1000
[  568.232717]  bfq_dispatch_request+0x1d7/0xd60
[  568.232731]  ? bfq_bfqq_handle_idle_busy_switch.isra.36+0x2cd/0xb20
[  568.232751]  blk_mq_do_dispatch_sched+0xa8/0xe0
[  568.232765]  blk_mq_sched_dispatch_requests+0xe3/0x150
[  568.232783]  __blk_mq_run_hw_queue+0x56/0x100
[  568.232798]  __blk_mq_delay_run_hw_queue+0x107/0x160
[  568.232814]  blk_mq_run_hw_queue+0x75/0x190
[  568.232828]  blk_mq_sched_insert_requests+0x7a/0x100
[  568.232844]  blk_mq_flush_plug_list+0x1d7/0x280
[  568.232859]  blk_flush_plug_list+0xc2/0xe0
[  568.232872]  blk_finish_plug+0x2c/0x40
[  568.232886]  ext4_writepages+0x592/0xe60
[  568.233381]  ? ext4_mark_iloc_dirty+0x52b/0x860
[  568.233851]  do_writepages+0x3c/0xd0
[  568.234304]  ? ext4_mark_inode_dirty+0x1a0/0x1a0
[  568.234748]  ? do_writepages+0x3c/0xd0
[  568.235197]  ? __generic_write_end+0x4e/0x80
[  568.235644]  __filemap_fdatawrite_range+0xa5/0xe0
[  568.236089]  ? __filemap_fdatawrite_range+0xa5/0xe0
[  568.236533]  ? ext4_da_write_end+0x13c/0x280
[  568.236983]  file_write_and_wait_range+0x5a/0xb0
[  568.237407]  ext4_sync_file+0x11e/0x3e0
[  568.237819]  vfs_fsync_range+0x48/0x80
[  568.238217]  ext4_file_write_iter+0x234/0x3d0
[  568.238610]  ? _cond_resched+0x19/0x40
[  568.238982]  new_sync_write+0x112/0x190
[  568.239347]  __vfs_write+0x29/0x40
[  568.239705]  vfs_write+0xb1/0x1a0
[  568.240078]  ksys_write+0x89/0xc0
[  568.240428]  __x64_sys_write+0x1a/0x20
[  568.240771]  do_syscall_64+0x5b/0x140
[  568.241115]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  568.241456] RIP: 0033:0x7fb5b02325f4
[  568.241787] Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b3 0f 1f 80 00 00 00 00 48 8d 05 09 11 2d 00 8b 00 85 c0 75 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 f3 c3 66 90 41 54 55 49 89 d4 53 48 89 f5
[  568.242842] RSP: 002b:00007ffcb12e2968 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[  568.243220] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb5b02325f4
[  568.243616] RDX: 0000000000000200 RSI: 000055698f2ad000 RDI: 0000000000000001
[  568.244026] RBP: 0000000000000200 R08: 0000000000000004 R09: 0000000000000003
[  568.244401] R10: 00007fb5b04feca0 R11: 0000000000000246 R12: 000055698f2ad000
[  568.244775] R13: 0000000000000000 R14: 0000000000000000 R15: 000055698f2ad000
[  568.245154] Modules linked in: xt_MASQUERADE(E) nf_conntrack_netlink(E) nfnetlink(E) xfrm_user(E) xfrm_algo(E) xt_addrtype(E) br_netfilter(E) bridge(E) stp(E) llc(E) overlay(E) vmw_vsock_vmci_transport(E) vsock(E) ip6table_filter(E) ip6_tables(E) xt_conntrack(E) iptable_mangle(E) iptable_nat(E) nf_nat(E) iptable_filter
[  568.248651] CR2: 0000000000000024
[  568.249142] ---[ end trace 0ddd315e0a5bdfba ]---


Regards,
Srivatsa
VMware Photon OS